Certain metal hydride (MH) alloy materials are capable of absorbing and desorbing hydrogen. These materials can be used as hydrogen storage media, and/or as electrode materials for fuel cells and metal hydride batteries including nickel/metal hydride (Ni/MH) and metal hydride/air battery systems.
When an electrical potential is applied between the cathode and a MH anode in a MH cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen to form a metal hydride (MH) and the electrochemical evolution of a hydroxyl ion. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron. The reactions that take place at the positive electrode of a nickel MH cell are also reversible. Most MH cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode.

In a MH cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felted, nylon or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide.
One particular group of MH materials having utility in MH battery systems is known as the ABx class of material with reference to the crystalline sites occupied by its member component elements. ABx type materials are disclosed, for example, in U.S. Pat. No. 5,536,591 and U.S. Pat. No. 6,210,498. Such materials may include, but are not limited to, modified LaNi5 type (AB5) as well as the Laves-phase based active materials (AB2). These materials reversibly form hydrides in order to store hydrogen. Such materials utilize a generic Ti—Zr—Ni composition, where at least Ti, Zr, and Ni are present with at least one or more of Cr, Mn, Co, V, and Al. The materials are multiphase materials, which may contain, but are not limited to, one or more Laves phase crystal structures.
Other ABx materials include the Laves phase-related body centered cubic (BCC) materials that are a family of MH alloys with a two-phase microstructure including a BCC phase and a Laves phase historically present as C14 as an example. These materials exhibit high density of the phase boundaries that allow the combination of higher hydrogen storage capacity of BCC and good hydrogen absorption kinetics and relatively high surface catalytic activity of the C14 phase. Many studies have been undertaken to optimize these materials. Young et al., Int. J. Hydrogen Energy, http://dx.doi.org/10.1016/j.ijhydene.2014.01.134 (article in press) describes a systematic study of these materials with a broad range of BCC/C14 ratio. These results reveal that while these materials have many desirable properties, the electrochemical storage capacity produced by these materials does not exceed 175 mAh/g.
Prior AB5 MH materials suffer from insufficient hydrogen-absorbing capabilities which equates to low energy density. This has made increasing the capacity of systems employing these materials exceedingly difficult. On the other hand, AB2 alloys commonly suffer from high cost and low high-rate performance.
Rare earth (RE) magnesium-based AB3- or A2B7-types of MH alloys are promising candidates to replace the currently used AB5 MH alloys as negative electrodes in Ni/MH batteries due in part to their higher capacities. While most of the RE-Mg—Ni MH alloys were based on La-only as the rare earth metal, some Nd-only A2B7 (AB3) alloys have recently been reported. In these materials, the AB3.5 stoichiometry is considered to provide the best overall balance among storage capacity, activation, high-rate dischargeability (HRD), charge retention, and cycle stability. The pressure-concentration-temperature (PCT) isotherm of one Nd-only A2B7 alloy showed a very sharp take-off angle at the α-phase [K. Young, et al., Alloys Compd. 2010; 506: 831] which can maintain a relatively high voltage during a low state-of-charge condition. Compared to commercially available AB5 MH alloys, a Nd-only A2B7 exhibited a higher positive electrode utilization rate and less resistance increase during a 60° C. storage, but also suffered higher capacity degradation during cycling [K. Young, et al., Int. J. Hydrogen Energy, 2012; 37:9882]. Another issue with known A2B7 alloys is that they suffer from inferior HRD relative to the prior AB5 alloy systems.
As such, there is a need for improved hydrogen storage materials and processes of their manufacture or activation. As will be explained herein below, the present invention addresses these needs by proving Laves phase-related BCC metal hydride alloys that for the first time exhibit greatly improved electrochemical properties. These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.